The multiphase voltage regulator is an electronic device used in several applications. For example, the multiphase voltage regulator may be used as a power supply in microprocessors for personal computers, workstations, servers, printers and other similar electronic equipment.
Electronic device microprocessors are at present being developed to meet the requirements sought by CPUs. Current CPUs require a high supply voltage precision, which can be approximately estimated in a voltage variation request of ΔV=(+/−0.8%) in steady state conditions, and of ΔV=(+/−3%) in transient conditions. Unfortunately, supply voltages can usually decrease to values of 1.1V while load currents can rise up to 100 A with possible slopes of 100 A/μs. However, an efficiency higher than 80% may be desired.
Research has identified the voltage regulator topology as a cheaper and more effective approach to meet the present requirements. The voltage regulator is realized in several configurations, and it allows a desired output voltage to be provided proportionally to the current required by the microprocessor. This feature is called a droop function or voltage positioning.
The droop function feature causes a device comprising a Current Sense circuit to be provided. This allows the current to be output by the device to be read or estimated. In general, the Current Sense circuit reads the current as a voltage drop across a resistance. This resistance can be a voltage regulator parasitic element. For example, in power switches the resistance may be the resistance Rds, on or the parasitic resistance of the inductance DCR. Alternatively, the resistance may be an element inserted in the circuit, such as a resistance Rsense, for example.
If a resistance Rsense is used, a very precise current reading is advantageously provided. Moreover, since constantan-made resistances are generally used, the reading is almost independent from temperature variations.
Nevertheless, this approach has for its known advantages the drawback of being a higher cost and for providing a reduced current conversion efficiency. If a regulator parasitic element is instead used, the approach is more economical since it exploits elements already in the regulator. But it also provides a less precise reading that is sensitive to single element errors, and to the variations produced therefrom according to the temperature of the regulator.
In particular, a current reading on the inductance parasitic resistance (DCR) has some advantages in terms of precision. The tolerance in this case is 5%. This is with respect to the reading on the resistance Rds,on of the low-side switch having a tolerance of about 30%.
Several voltage regulators with reading systems comprising the droop function implemented to obtain the current measure of the voltage regulator module (VRM) inductors are available. An example of a voltage regulator in a standard dual-phase configuration, also called a multiphase DC-DC converter, is represented in FIG. 1. The voltage regulator has for each module or phase a dc-dc switch comprising a pair of field-effect transistors, HS and LS, interposed in series between an input voltage Vin and a ground voltage. An inductance L is interposed between an intermediate node of the two transistors HS and LS and the output.
The output current of each module is added to the current coming from the other modules. This defines the output current of the voltage regulator IOUT. The voltage regulator has, for each module, a Current Sense circuit comprising a parasitic resistance RL of the inductance L, and a filtering network comprising a capacitor C and a resistance R providing the output signal at the input of the inductance L. In particular, the value of the resistance R is defined so that for each circuit the relation R*C=L/RL applies.
Therefore, the output current of the first switching element has a sawtoothed configuration as indicated by Phase 1 current in the graph of FIG. 2. Also, the output current IOUT of the voltage regulator, which is the sum of the two current signals, has a sawtoothed configuration and a period corresponding to T/2.
In general, a multiphase voltage regulator with N switches has an output current with a sawtoothed configuration with a period corresponding to T/N. Nevertheless, the voltage regulators require a control circuit of the phase displacements between the input currents of the N switches. This allows a feedback operation on the switches.
A known device is described in U.S. Pat. No. 5,982,160 to Walters et al., where an R-C current sense network is connected in parallel to each output inductor of the voltage regulator, as emphasized in FIG. 3. Values of the current sense circuit, i.e., R and C, are determined by the time constant matching. That is, by the equality between the time constant of the circuit RL-L or coil network and of the current sense R-C circuit.
Moreover, the current sense circuit current signal of each R-C network is analyzed together with the output signal through a resistance Rg by a controller. In the case represented in FIG. 3, a circuit is shown for estimating the output current of the dual-phase voltage regulator emphasizing that the controller for each R-C network analyses two pins. Therefore, by generalizing, for N phases or modules the controller analyzes 2N pins.
However, the voltage regulator according to the provided approach, although advantageous under several aspects, has some drawbacks. The controller produces a current or voltage signal which will be proportional to the resistance RL, RG, and to the output current IOUT. Therefore, the good quality of the produced signal will depend on the good quality of the inserted components, and on the variations thereof according to the temperature. This regulator requires N R-C networks, one for each phase. The capacitor CCS is generally of the COG type, and is particularly expensive. The controller requires 2*N pins to read the total current.
This involves a regulator circuit complexity and the need for a considerable surface for the in-silicon-chip integration. Moreover, the N R-C networks can insert some delays in output signals that could invalidate the voltage regulator reliability.
A further known multiphase voltage regulator is described in U.S. Pat. No. 6,683,441 B2 to Schiff et al. and is shown in FIG. 4 in a dual-phase example. In this approach, the voltages of the two nodes, PHASE1 and PHASE2, are added, preferably by a resistance Rp, and sent to an operational amplifier which further receives the regulator output voltage.
The operational amplifier has feedback, an R-C network, or a circuit sense, comprising a resistance RCS and a capacitor CCS located in parallel and defined so that RCS is determined by the time constant matching between the time constant RL-L e RCS-CCS. This is done to output a voltage VCS proportional to the regulator output current IOUT, according to the relation:
      V    CS    =            V      OUT        -                                        R            L                    ·                      R            CS                                    R          P                    ·              I        OUT            
A controller then analyses the two input signals and the output signal of the operational amplifier, thus requiring three pins. To obtain a signal being directly proportional to the regulator output voltage Vout, it is necessary to use, after the operational amplifier, a further amplifier for removing the constant term Vout, according to the relation:
      V    DROOP    =                    V        OUT            -              V        CS              =                                        R            L                    ·                      R            CS                                    R          P                    ·              I        OUT            
Therefore, the present approach, though responding to the problem and being advantageous under several aspects, has some drawbacks. This includes requiring the use of N external resistances RP for the adder node, a sense circuit comprising the R-C network for the time constant matching and an amplifier, while the controller requires three pins for the reading. Moreover, to proportionally determine the regulator output voltage, a second amplifier needs to be available.
In voltage regulators that are currently available, the presence of more precise components to meet the need for more stringent specifications, poses further problems. In particular, the use of smaller and smaller inductors with lower and lower parasitic resistances RL clashes with the presence of the parasitic resistances on the application boards because of the unavoidable resistances due to the copper tracks.
As emphasized in FIG. 5, for a dual-phase regulator, the presence of the parasitic resistances Rp1 in series with the resistances RL requires a reading circuit that can read the information directly across the coil network. That is, the inductance L in series with the resistance RL avoids the parasitic resistances RPi of the board tracks. This reading typology is called fully differential.
The current reading described in U.S. Pat. No. 5,982,160 to Walter et al., shown in FIG. 3, is a fully differential reading. Nevertheless, this approach requires, as already emphasized, N R-C networks where N is the number of phases and 2*N is the number of pins for the total current reading.
With the case indicated in U.S. Pat. No. 6,683,441 and as shown in FIG. 4, a fully differential current reading can not be provided since this approach takes as a reference the output voltage VOUT, summing both phases. Therefore, if there were different parasitic resistances for the two phases, an error would be committed in the current measure.